The present invention relates generally to motor vehicle radar detectors and, more particularly, to radar detectors which provide improved sensitivity and a satisfactory level of "false alarms" by means of signal amplitude detection.
Radar signals have been commonly used by police for some time to determine the speed of motor vehicles. In response to radar speed monitoring and to signal motor vehicle operators when such monitoring is taking place, police radar detectors have likewise been used for almost a coincident period of time. Radar detectors sense radar signals and alert motor vehicle operators of their presence by audible and/or visual alarms. To ensure that radar detectors advise operators of radar monitoring operations at the earliest possible time, the detectors must be sufficiently sensitive to detect weak radar signals transmitted from as great a distance as possible. It is also important to minimize the number of "false alarms", i.e. alarms generated in response to signals/noise other than speed monitoring radar signals, produced by radar detectors such that an operator can rely on radar alerting signals generated by the detector. Unfortunately, detection of weak signals and minimizing false alarms present conflicting goals.
Currently available police radar detectors typically employ swept superheterodyne receivers which include FM discriminators whose output signals are filtered and passed to voltage comparators to detect the presence of radar signals. In an attempt to achieve both the weak signal detection and low false alarm goals, elaborate filters have been used that are matched to the "s-shape" of the detected FM signal which is generated as the signal sweeps through the intermediate frequency (IF) amplifier. Another approach is to perform digital signal processing on the output signal from the discriminator to improve post-discriminator dynamic range. Unfortunately, all known approaches tend to be complex and expensive and the detectors still are made up of an FM discriminator, a filter which is possibly complex and circuitry to finally make a radar signal present or no radar signal present decision.
Further, all such known detectors also employ an inordinately wide IF bandwidth in comparison to desired matched filter considerations, possibly due to signal detection considerations. While matched filter bandwidth is approximately the square root of the sweep rate of the swept superheterodyne receiver, known detectors use a substantially wider IF bandwidth. With wider IF bandwidth, the "s-curve" persists for a relatively longer period of time and therefore its energy is concentrated at lower frequencies making it easier to detect in the presence of broadband noise from the discriminator. Conversely, as the IF bandwidth is narrowed the "s-curve" occurs faster occupying a wider bandwidth, and is progressively more difficult to detect in the presence of noise occupying a similar bandwidth. Further confounding the recognition task is the fact that the desired "s-curve" is a bipolar signal buried in bipolar noise. Still another difficulty is the inherent nature of the FM limiter/discriminator: improving input signal strength is not manifested solely by increased output signal amplitude but also by suppression of the noise component. Recognizing improved signal to noise ratio arising from suppressed noise is a complex task tending to require expensive hardware.
Accordingly, there is a need for an improved radar detector which operates effectively with IF bandwidth more closely approximating the matched filter bandwidth and which overcomes the foregoing problems associated with conventional radar detector techniques utilizing FM discriminators whose output signals are filtered and passed to voltage comparators or other circuitry to detect the presence of radar signals.